1. Field of the Invention
Embodiments of the present invention generally relate to the fabrication of photovoltaic cells and particularly to the formation of layers on a substrate by use of an electrochemical deposition process.
2. Description of the Related Art
Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or polycrystalline wafers. Because the amortized cost of forming a silicon-based solar cells to generate electricity is higher than the cost of generating electricity using traditional methods, there has been an effort to reduce the cost to form solar cells.
FIGS. 1A and 1B schematically depicts a standard silicon solar cell 100 fabricated on a wafer 110. The wafer 110 includes a p-type base region 101, an n-type emitter region 102, and a p-n junction region 103 disposed therebetween. An n-type region, or n-type semiconductor, is formed by doping the semiconductor with certain types of elements (e.g., phosphorus (P), arsenic (As), or antimony (Sb)) in order to increase the number of negative charge carriers, i.e., electrons. Similarly, a p-type region, or p-type semiconductor, is formed by the addition of trivalent atoms to the crystal lattice, resulting in a missing electron from one of the four covalent bonds normal for the silicon lattice. Thus the dopant atom can accept an electron from a neighboring atoms' covalent bond to complete the fourth bond. The dopant atom accepts an electron, causing the loss of half of one bond from the neighboring atom and resulting in the formation of a “hole”.
When light falls on the solar cell, energy from the incident photons generates electron-hole pairs on both sides of the p-n junction region 103. Electrons diffuse across the p-n junction to a lower energy level and holes diffuse in the opposite direction, creating a negative charge on the emitter and a corresponding positive charge builds up in the base. When an electrical circuit is made between the emitter and the base and the p-n junction is exposed to certain wavelengths of light, a current will flow. The electrical current generated by the semiconductor when illuminated flows through contacts disposed on the frontside 120, i.e. the light-receiving side, and the backside 121 of the solar cell 100. The top contact structure, as shown in FIG. 1A, is generally configured as widely-spaced thin metal lines, or fingers 104, that supply current to a larger bus bar 105. The back contact 106 is generally not constrained to be formed in multiple thin metal lines, since it does not prevent incident light from striking solar cell 100. Solar cell 100 is generally covered with a thin layer of dielectric material, such as Si3N4, to act as an anti-reflection coating 111, or ARC, to minimize light reflection from the top surface of solar cell 100.
The fingers 104 are in contact with the substrate are adapted to form an ohmic connection with doped region (e.g., n-type emitter region 102). An ohmic contact is a region on a semiconductor device that has been prepared so that the current-voltage (I-V) curve of the device is linear and symmetric, i.e., there is no high resistance interface between the doped silicon region of the semiconductor device and the metal contact. Low-resistance, stable contacts are critical for the performance of the solar cell and reliability of the circuits formed in the solar cell fabrication process. Hence, after the fingers 104 have been formed on the light-receiving surface and on the backside, an annealing process of suitable temperature and duration is typically performed in order to produce the necessary low resistance metal silicide at the contact/semiconductor interface. A backside contact completes the electrical circuit required for solar cell to produce a current by forming an ohmic contact with p-type base region of the substrate.
Widening the current carrying metal lines (e.g., fingers 104) on the light-receiving surface of the solar cell lowers the resistance losses, but increases the shadowing losses due to the reduced effective surface area of the light-receiving surface. Therefore, maximizing solar cell efficiency requires balancing these opposing design constraints. FIG. 1C illustrates a plan view of one example of a top contact structure 135 for a conventional pin-up module (PUM) cell, wherein the finger width and geometry have been optimized to maximize cell efficiency for the cell. In this configuration, a top contact structure 135 for a PUM cell is configured as a grid electrode 138, which consists of a plurality of various width finger segments 135A. The width of a particular finger segment 135A is selected as a function of the current to be carried by that finger segment 135A. In addition, finger segments 135A are configured to branch as necessary to maintain finger spacing as a function of finger width. This minimizes resistance losses as well as shadowing by finger segments 135A.
Traditionally, the current carrying metal lines, or conductors, are fabricated using a screen printing process in which a silver-containing paste is deposited in a desired pattern on a substrate surface and then annealed. However, there are several issues with this manufacturing method. First, the thin fingers of the conductors, when formed by the screen printing process, may be discontinuous since the fingers formed using a metal paste do not always agglomerate into a continuous interconnecting line during the annealing process. Second, porosity present in the fingers formed during the agglomeration process results in greater resistive losses. Third, electrical shunts may be formed by diffusion of the metal (e.g., silver) from the contact into the p-type base region or on the surface of the substrate backside. Shunts on the substrate backside are caused by poor definition of backside contacts such as waviness, and/or metal residue. Fourth, due to the relatively thin substrate thicknesses commonly used in solar cell applications, such as 200 micrometers and less, the act of screen printing the metal paste on the substrate surface can cause physical damage to the substrate. Lastly, silver-based paste is a relatively expensive material for forming conductive components of a solar cell.
One issue with the current method of forming metal interconnects using a screen printing process that utilizes a metal particle containing paste is that the process of forming the patterned features requires high temperature post-processing steps to densify the formed features and form a good electrical contact with the substrate surface. Due to the need to perform a high temperature sintering process the formed interconnect lines will have a high extrinsic stress created by the difference in thermal expansion of the substrate material and the metal lines. A high extrinsic stress, or even intrinsic stress, formed in the metal interconnect lines is an issue, since it can cause breakage of the formed metallized features, warping of the thin solar cell substrate, and/or delamination of the metallized features from the surface of the solar cell substrate. High temperature processes also limit the types of materials that can be used to form a solar cell due to the breakdown of certain materials at the high sintering temperatures. Also, screen printing processes also tend to be non-uniform, unreliable and often unrepeatable. Therefore, there is a need to form a low stress interconnect line that forms a strong bond to the surface of the substrate.
Another approach to forming very thin, robust current carrying metal lines on the surface of a solar cell substrate involves cutting grooves in the surface of the substrate with a laser. The grooves are subsequently filled by an electroless plating method. However the laser-cut grooves are a source of macro- and micro-defects. The laser-cut edge is not well defined, causing waviness on the finger edges, and the heat of the laser introduces defects into the silicon.
Other traditional methods of forming the metal lines (e.g., fingers 104) in a metal interconnect structure are performed by use of expensive multistep processes to form a desired pattern of metallized features on the substrate surface. These processes can include deposition of a blanket metal film, performing a masking step, such as a lithography type steps, to form a desired pattern and then performing electroless or electroplating processes to build up the thickness of the formed metal lines. In one example, the process of forming metal line includes the steps of: 1) depositing a blanket metal layer over the surface of the substrate, 2) depositing a resist layer over the metal layer, 3) exposing portions of the resist layer to some form of radiation, 4) developing the resist, 5) electroplating the metal layer in the exposed portions of the blanket metal, 6) removing the resist layer from the surface of the substrate, and then 7) etching away the blanket metal layer between the plated areas on the substrate surface. In another example, known as the ink jet printing method, the process of forming metal line includes the steps of: 1) depositing a blanket metal layer over the surface of the substrate, 2) depositing an ink layer over various portions of the blanket metal layer, 3) drying the ink layer, 4) electroplating the metal layer in the exposed portions of the blanket metal, 5) removing the ink layer from the surface of the substrate, and then 6) etching away the blanket metal layer between the plated areas on the substrate surface. These conventional processes require a number of processing steps that make the cost to produce the substrate fairly expensive and increase the chance of the substrate being misprocessed, thus causing substrate scrap and waste.
In recent years the solar cell industry has been working ways to form a low cost flexible solar cell for use in varied electrical applications, such as computers, smart cards, curved building surfaces, clothing, retractable satellite solar arrays, portable electronic devices, and cell phones. Flexible solar cells are attractive since they can be formed inexpensively using a high speed production processes, such as roll-to-roll manufacturing methods. Flexible substrate can be constructed from polymeric materials, such as a polyimide (e.g., KAPTON™ by DuPont Corporation), polyethyleneterephthalate (PET), polyacrylates, polycarbonate, silicone, epoxy resins, silicone-functionalized epoxy resins, polyester (e.g., MYLAR™ by E.I. du Pont de Nemours & Co.), APICAL AV manufactured by Kanegaftigi Chemical Industry Company, UPILEX manufactured by UBE Industries, Ltd.; polyethersulfones (PES) manufactured by Sumitomo, a polyetherimide (e.g., ULTEM by General Electric Company), and polyethylenenaphthalene (PEN). In some cases the substrate can be constructed from a metal foil, such as stainless steel that has an insulating coating disposed thereon. Alternately, flexible substrate can be constructed from a relatively thin glass that is reinforced with a polymeric coating. In most low cost applications flexible substrates use a material that cannot be to brought to temperatures exceeding 250° C. and thus the metallizing process must be performed at temperatures well below these temperatures. Also, typical conventional deposition processes will not allow a conductive layer to selectively formed on the surface of the substrate. Therefore, there is a need for a low cost process that can rapidly and selectively form a conductive on the surface of a flexible substrate. Also, there is a need for a low cost method of forming a contact structure for solar cells that can be rapidly formed the have an low cost of ownership (CoO).